This invention relates to two stage filters for use in a liquid lubricant circulating system of an internal combustion engine and more particularly to an improved two stage filter incorporating a primary fine filtering media and a secondary coarser filtering media, such filter constructed and arranged so as to provide higher cleaning efficiencies than prior like filters.
Dual element filters or two stage filters of different types are known. Mould et al. U.S. Pat. No. 2,998,138 and Hultgren U.S. Pat. No. 3,370,708 each shows a filter incorporating a pair of concentric, cylindrical filter elements. Humbert et al. U.S. Pat. No. 3,467,256 reveals a screw-on throwaway type filter having a casing with a pair of concentric filter elements therein.
Dison U.S. Pat. No. 4,038,189 shows a dual filter having a primary fine mesh filter element, a secondary coarser mesh filter element in parallel with the primary filter element and a plurality of bypass valves which are normally closed to block flow through the secondary filter element, but which open at an elevated fluid pressure caused by clogging of the primary filter element to allow flow through the secondary filter element.
Ball U.S. Pat. No. 3,268,077 pertains to a dual element, dual valve filter having a first filter element 51 and a second filter element 48. When the second filter element becomes clogged, a valve is opened to bypass oil about both filter elements.
Neely U.S. Pat. No. 3,269,541 pertains to a dual element dual valve filter assembly incorporating concentric filter media. In one embodiment, the inner filter media includes a flat resilient inlet valve which opens when a predetermined pressure differential is attained within the filter housing to permit passage of oil through the inner filter media.
The prior art discloses dual element filters which include separate valve elements to control flow through the secondary filter element. When the valve is opened, the flow of liquid bypasses the filter elements and unfiltered liquid is returned to the lubricating circulating system. The present invention provides a dual element filter assembly, which utilizes no separate valve member to control flow through the secondary filter element. Rather, the inventive filter incorporates a very fine first filter element and a high density coarser secondary filter element, which changes shape to accommodate increased oil flow, without opening an unfiltered path for the oil to return to the lubricant circulating system.
An object of the present invention is to provide an improved two stage liquid filter wherein deficiencies and disadvantages of prior constructions are obviated.
Another object of the present invention is to provide an improved two stage filter having a housing containing a primary fine filter and a secondary coarse filter, the secondary coarse filter deforming to relieve a portion of the differential pressure due to clogging of the primary filter, without opening an unfiltered path for the oil to return to the lubricant circulating system. Other objects and advantages of the present invention will become more apparent hereinafter.